Gathering petrophysical, geophysical and well production information through well logging techniques using instruments suspended with stranded cables is well known and widely practiced. Typical measurements made by such methods include various types of geophysical and petrophysical measurements as well as various types of well production information including, but not limited to, formation pressure, flow rate, cement status, water flow, corrosion and the response of the well bore environment to sundry electrical, acoustic, nuclear and magnetic stimuli.
The conventional cable used in well-logging is a stranded multi-conductor cable which includes a layer of armor strands. One version of this cable has a core comprised of six outer conductors cabled around a single center conductor and embedded in a neoprene matrix. The outer conductors are formed by copper wire strands twisted around a single center strand. Each conductor is covered with a layer of suitable insulation material. Although the neoprene matrix fills substantially all the voids between conductors within the cable core, voids still may exist within the conductors themselves between and about the strands. The core is surrounded by a jacket of insulating material. The jacket is enclosed by a first and a second layer of armor strands. The core may include electrical conductors and/or optical fibers and electrical insulating and mechanical protecting sheaths immediately surrounding the electrical conductors or the optical fibers. In a second version of this cable, the jacket between the core and the armor strands is made of a thermoplastic material such as Polyethylene or Ethylene Propylene Copolymer (EPC). This thermoplastic material is such that it allows strands to embed into the jacket material. The armor strands lie in grooves generated on the periphery of the jacket and the grooves help to maintain the armor strands in a close relationship with the jacket/core. When tension is applied to the cable, the thermoplastic material fills the interstices between the armor strands, the armor strands embed deeper into the jacket material, and over time, the cable becomes permanently elongated.
Although use of the foregoing cables is highly satisfactory for many well logging operations, use of either cable in a well containing substantial amounts of low molecular weight hydrocarbons, such as methane gas, involves a substantial risk of failure in the cable and/or the cable terminations when the cable is rewound after a logging job. Due to the borehole depth and a wellbore temperature in excess of 150.degree. C., which is quite common, the gas can permeate the matrix of the cable and the insulation materials of the conductors due to a phenomenon that may be called activated diffusion. The permeation causes pressure buildup and gas entrapment in the conductor voids. As the cable is removed from the well and wound back upon the drum at the surface, release of the entrapped gas is only accomplished through bleed out at the terminated ends of the conductors, or outright rupture of the insulation materials themselves. In either case, releasing the gas may result in an undesirable cable failure due to an electrical short.
Other characteristics in a borehole environment, in particular downhole pressure, can greatly affect cable performance. Extremely high pressure can cause the migration of borehole fluid inside the cable. This migration of fluid will directly affect the transfer of data from the downhole logging tool to the surface. In addition, downhole pressure can enter the cable and damage the conductor insulation.
For the foregoing reasons, there is a need for an apparatus which isolates pressure from the surface environment while simultaneously permitting the entrance and movement of a cable for downhole logging.